For me personally in 11.10 I tried half a dozen different posted fixes checking and un-checking things various config editors and Ubuntu-Tweak was the only thing that actually worked. In the current version it is under Tweaks > Session Control
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DennisNov 30 '11 at 2:11

For Ubuntu 12.04 (Tested on 12.04, should also work on other versions)

The good thing about this is, You don't need to install dconf-tools, though it is better to install it, which will give you a graphical settings editor called dconf-editor to change many other features